Laser processing method

1. A laser processing method for applying a laser beam to a back surface of a substrate with a device formed on a front surface thereof and including an electrode pad, to form a fine hole in the substrate that reaches the electrode pad, the method comprising: a laser beam applying step of applying the laser beam to the back surface of the substrate to form a fine hole in the substrate at a position corresponding to the electrode pad; a detecting step of detecting, through a first optical path, first plasma light having a first wavelength emitted from the substrate at the same time that the fine hole is formed in the substrate by the laser beam applied thereto; and, through a second optical path, second plasma light having a second wavelength different from the first wavelength emitted from the electrode pad; and a laser beam irradiation finishing step of stopping application of the laser beam when the second plasma light is detected in the detecting step, wherein, in the laser beam applying step, a peak power density of the laser beam to be applied is set in a range from 175 GW/cm 2 or less to 100 GW/cm 2 or more.

2. The laser processing method according to claim 1 , wherein, in the laser beam applying step, a peak power density of the laser beam to be applied is set in a range from 150 GW/cm 2 or less to 125 GW/cm 2 or more.

3. The laser processing method according to claim 1 , wherein the first wavelength is in a range from approximately 660 nm to 680 nm, and the second wavelength is in a range from approximately 510 nm to 520 nm.

4. The laser processing method according to claim 1 , wherein the first wavelength is approximately 670 nm, and the second wavelength is approximately 515 nm.

5. The laser processing method according to claim 1 , wherein the laser beam irradiation finishing step further comprises, determining a light intensity of the second wavelength and stopping the application of the laser beam when the light intensity reaches a threshold value.